1. Field of the Invention.
The present invention relates to r.f. modems, and more particularly to a constant carrier watchdog circuit for an r.f. modem using frequency shift keying.
2. Description of the Prior Art.
In telecommunication systems many users access the same data channel on a time share basis. To access a broadband communications bus an r.f. modem, such as the Sytek Model 20/100, manufactured by Sytek, Inc. of Mountain View, Calif. is located at the users terminal. The r.f. modem detects when an r.f. channel is not busy and transmits a packet of data onto the r.f. channel. The data packet is addressed to a particular terminal with its own r.f. modem, and the r.f. modem passes the data packet to that terminal.
A plurality of such r.f. modems are networked together to form a telecommunication system, and each r.f. modem has the capability of accessing one of a plurality of data channels. At power up each r.f. modem selects one of the data channels via a PCU instruction which loads a count into a counter in the phase-locked loop frequency synthesizer. The count determines the frequency of the carrier for that r.f. modem, the frequency determining the data channel. Each r.f. modem also has a unique address on that data channel. Thus, by selecting the appropriate data channel and r. f. modem address, any terminal can communicate with any other terminal or computer host on the network.
The data is transmitted in the form of "1"s and " "s which cause a frequency shift of the carrier frequency. For example, to transmit data the carrier frequency may be turned on for "0"s, and shifted up or down for "1"s. However, from time to time due to an anomaly in the r.f. modem the carrier gets turned on when no data is being transmitted. When the r.f. modem asserts carrier on the communications channel, all other users on that data channel are locked off the network. For short periods of time this may not present a problem, but for longer periods, typically 8-10 seconds, this may result in data loss to other users.
The isolation of the offending r.f. modem once a user reports a problem requires a systematic turning off and on of each node of the network starting from the head-end and working down the branches. Once the node and branch from that node which contains the anomalous r.f. modem is discovered, then that branch is disconnected to allow the rest of the network to resume normal operation. Finally, each r.f. modem on the branch is then checked to discover the malfunctioning unit. This is a time consuming process which can take up to eight man hours to complete.
Therefore, what is desired is a means for instantaneously identifying an r.f. modem when it malfunctions in this manner.